It is known per se from the prior art for blade rows of turbines to be equipped with shrouds. The shrouds may in this case, for example, be in the form of outer shrouds on the outer circumference of a blade row. The shrouds are also generally in the form of split shrouds, with the relevant shroud being subdivided on the circumference of a blade row into a large number of shroud elements corresponding to the number of blades in the blade row. Each blade then has one associated shroud element, with the blade and the shroud element generally being formed integrally. The shroud elements are generally in the form of platforms and extend essentially at right angles to the blade longitudinal direction. When the blades are arranged in a row on the circumference of a turbomachine, in a known manner, the shroud elements of the blades are thus adjacent to one another and thus form a shroud which is closed on the circumference. In the case of outer shrouds, the respective shroud element is located at the blade tip, that is to say at the free end of the blade section of the blade.
A shroud may be arranged on a blade row for various reasons. Firstly, the arrangement of a shroud makes it possible to improve the vibration behavior of a blade. Adjacent blades are coupled to one another by the split shroud elements in the area of the blade tips or in the area of the blade root. This on the one hand increases the oscillatory mass of the blade and thus changes the natural frequency behavior. A shroud which is arranged at the blade tips also acts like an additional form of clamping for the blade sections of the blade, thus fundamentally improving the oscillatory behavior. In addition, a shroud also makes it possible to increase the damping, since, when the blade is stimulated to oscillate, relative movements occur between the contact surfaces between the shroud elements, thus converting kinetic energy to thermal energy.
A further aspect is that the arrangement of shrouds reduces the leakage of the main flow. This is because the shroud forms a virtually closed flow channel wall which is sealed with respect to the housing located behind it, or else with respect to the shaft. In consequence, virtually no fluid from the main flow enters the intermediate space between the shroud and the housing, and thus cannot escape as a leakage flow through gaps in the housing, either.
The outer shroud elements of an outer shroud for a rotor are normally arranged at the blade section tip such that the center of gravity of the outer shroud is balanced in relation to the respective blade root. Since, however, modern blade sections are generally designed to be twisted and also curved in some cases, this means that the shroud elements are not symmetrically balanced. This means that one platform section of the shroud element, which extends on one side of the blade section (for example the pressure face), is not equal to the other platform section of the shroud element, which extends on the other side of the blade section (for example the suction face). In particular, the platform sections often have unequal projection lengths. This nonuniformity of the platform sections leads to bending torques of different magnitude on the pressure-side and suction-side platform sections when the blade is used in a rotor, owing to the centrifugal forces acting on the platform sections. The different bending torques in turn lead to different elastic deformation of the platform sections on the pressure side and suction side. This situation is illustrated in FIG. 4. The pressure-side platform section in FIG. 4 has a larger projection length than the suction-side platform section, and is subject to a greater bending torque during rotation, because of the higher mass and the longer lever arm, and this in turn leads to greater elastic deformation of the pressure-side platform section. The pressure-side platform section is in consequence bent to a greater extent than the suction-side platform section of the adjacent shroud element, thus resulting in a gap being produced between the pressure-side platform section and the suction-side platform section, through which fluid from the main flow can escape in the manner illustrated in FIG. 4. The escape of fluid through the resultant gap is further exacerbated here because the fluid is forced or pressed into the gap as a result of the rotation direction in the direction of the pressure face, in a similar manner to a blade effect.
In addition to the high bending forces, the shrouds, particularly for turbine stages, are often additionally subject to very high temperatures from the main flow. The combined load has a negative influence on the time/creepage behavior of the platform sections. Those platform sections which have a longer free projection length and in consequence are subject to a greater bending torque during operation are also deformed by an increased creepage behavior. The creepage behavior is in turn directly coupled to the projection length, and leads to an increase of the effect illustrated in FIG. 4.
As the component age increases, an increased amount of fluid escapes from the main flow through the gap as the gap size increases. Particularly in the turbine area, the fluid in the main flow is in this case at a very high temperature, resulting in a dramatic rise in the material temperature both on the rear face of the shroud and on the adjacent components. On the one hand, this once again leads to an increase in the creepage behavior described above, and on the other hand leads to an increased temperature load on the adjacent components. In some cases, even local material overheating occurs, so-called hot spots. In any case, this effect leads in some cases to a very considerable shortening of the life of virtually all of the components which are affected. A blade whose shroud has reached a specific creepage deformation is thus nowadays replaced at an early stage after only a short life, in order in this way to prevent further damage being caused.